Pyrolysis processes, such as steam cracking, can be utilized for converting saturated hydrocarbons to higher-value products such as light olefin, e.g., ethylene and propylene. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value heavy products, such as pyrolysis tar. When the pyrolysis is steam cracking, the pyrolysis tar can be identified as steam-cracker tar (“SCT”).
SCT generally contains relatively high molecular weight molecules, conventionally called Tar Heavies (“TH”). Catalytic hydroprocessing of undiluted SCT leads to significant catalyst deactivation. For example, significant reactor coking is observed when hydroprocessing SCT at a temperature in the range of from 250° C. to 400° C., at a pressure in the range of 5400 kPa to 20,500 kPa, using (i) a treat gas containing molecular hydrogen and (ii) at least one catalyst containing one or more of Co, Ni, or Mo. The coking has been attributed to the presence of TH in the SCT.
It is conventional to lessen the amount of coking by hydroprocessing the tar in the presence of a utility fluid, e.g., a solvent having significant aromatics content. The hydroprocessed tar product generally has a decreased viscosity, decreased atmospheric boiling point range, and an increased hydrogen content over that of the SCT, resulting in improved compatibility with fuel oil blend-stocks. Conventional processes for SCT hydroprocessing, disclosed, e.g., in U.S. Pat. Nos. 2,382,260 and 5,158,668; and in PCT Patent Application Publication No. WO2013/033590, involve recycling a portion of the hydroprocessed tar for use as the utility fluid.
It can be desirable to maintain the SCT at a temperature ≦400° C. before and during hydroprocessing, and in any pre-heat stages upstream of the hydroprocessing reactor. Conventional methods for doing so are disclosed in PCT Patent Application Publication No. WO2013/033582, which describes combining the treat gas and utility fluid, pre-heating the utility fluid-treat gas mixture, and then adding the SCT to the heated mixture upstream of the hydroprocessing reactor. When utilizing hydroprocessed tar as a utility fluid, it is conventional to combine the hydroprocessed tar with a supplemental utility fluid obtained from an external source, e.g., Steam Cracked Naphtha (“SCN”), to further lessen increases in reactor pressure drop as can occur from the formation of coke deposits in the hydroprocessing reactor and/or SCT pre-heating equipment.
Since the supplemental utility fluid is a valuable product of the steam cracking process, there is a need for SCT hydroprocessing processes having a decreased need for supplemental utility fluid, particularly for such processes as can be operated over a broad SCT compositional range and/or a range of hydroprocessing temperature and pressure.